Correction of blood pressure measurements in invasive liquid-filled systems

ABSTRACT

The invention relates to a method for the correction of measured value falsifications in invasive pressure measurements with a fluid-filled system, in which the measured pressure is passed via the fluid-filled system to an external pressure transducer, which converts the pressure signal into an electrical signal. To provide a method and a device for invasive pressure measurement with fluid-filled systems which are improved with respect to the correction of measured value falsifications, are cost-effective and versatile in their use, the electrical signal is passed through an analog/digital converter and the digitized signal is fed to a signal analyzing and processing unit, which operates on the basis of a digital Fourier analysis. Furthermore, a heartbeat-related or segmental analysis is carried out and the signal is combined with empirically determined correction data on the basis of the analysis. The correction data are emitted as Fourier coefficients and the signal corrected by the signal analyzing and processing unit is passed to an output and/or evaluating unit.

BACKGROUND

The invention relates to a method and a device for the correction ofmeasured value falsifications in invasive pressure measurements with afluid-filled system, in which the measured pressure is passed via thefluid-filled system to an external pressure transducer, which convertsthe pressure signal into an electrical signal.

Fluid-filled systems have been used for decades in connection withinvasive pressure measurement for intravenous and intraarterial pressuremeasurement. Such systems, also referred to as catheters, are frequentlyused in invasive cardiology, intensive medicine and in anesthesia, wherethey are used for exact pressure measurement. Use is particularlyappropriate for impedance measurements on the arterial system of vesselsor for derivatives of pressure with respect to time (dp/dt) formeasuring the isovolumetric force of contraction or relaxation disordersof the ventricles. For this purpose, it must be possible to analyzeresonances of the original pressure signals of up to approximately 30 Hzfaithfully with respect to the original, that is faithfully with respectto phase and amplitude.

In invasive catheter diagnosis, the pressure measurement at a specificlocation in the circulation takes place via a fluid-filled system with apressure transducer applied externally (i.e. outside the patient'sbody). Depending on the length, cross section, setup and elasticmaterial properties of these systems, different resonances, attenuationsand energy losses of the input pressure signal occur at the tip of thecatheter.

U.S. Pat. No. 4,232,373 discloses a correction method for measurementdata of a fluid-filled cardiac catheter, in which the periodicallyrecorded signal is converted into an electrical signal, digitized andbranched. Part of the signal is first passed to a correction unit andsubsequently passed to a filter, while the other part is passed to thefilter in an uncorrected form and with a delay. In the filter, the twoparts of the signal are brought together and the corrected signal isoutput.

In the manuscript “Characterization of laser-induced pressure transientsby means of piezoelectric PVDF-films” by S. Lohmann et al; Proc.SPIE2624; 83-92; (1995), there is described, inter alia, the correction oflaser-induced pressure waves in piezoelectric films. In this case, adescription is given of the correction of a voltage signal emitted bythe film by means of a Fourier transformation, in which the signal istransformed into the frequency domain and is corrected in the frequencydomain by means of a correction value calculated in an algorithm.Subsequently, an inverse transformation into the time domain is carriedout.

To avoid falsifications along the transmission path, the pressuretransducer has been integrated into the tip of the catheter and theconverted signal led out of the body via an electric line. This solutionis known as a tip pressure sensor catheter. A disadvantage of this formof pressure measurement is that tip pressure sensor catheters are veryexpensive and have only a very restricted range of variations withrespect to shape and size. Therefore, it has only been possible for thissolution to be established in the scientific sector to a limited extent.

A further possible way of compensating for measured value falsificationsis to consider the system as a simple forced oscillation in the physicalsense and to carry out a correction of the transmission function of thesystem of the 2nd order after determination of the resonant frequencyand the attenuation coefficient by means of an analog electric circuitor a corresponding numerical algorithm. The disadvantages of thisapproach are that the consideration as a system of the 2nd order is agreat simplification of the actual physics of the system, in whichmultiple resonances can occur particularly in the case of relativelycomplex systems. The transmission function is, in principle, to be newlydetermined for each actual system, even when there are customary andfrequent changes such as exchanging the catheter in the system, it beingproblematical to determine the transmission function by means of aflushing test or square-wave test on the patient. The transmissionfunction is, furthermore, dependent on the elasticity of the system andthis in turn is dependent on the filling pressure, the gases dissolvedin the fluid and material properties of the system. Finally, thesesystems are very complicated to operate.

A further procedure introduced on the market is the use of systems whichhave been specially configured and optimized in terms of fluid mechanicsby in-vitro test studies and which comprise a pressure transducer, atube, a three-way cock, an array of cocks, a catheter and possibly anattenuator. A disadvantage of this method is that the test effort isvery great and that, in invasive cardiology, an extremely wide varietyof systems are used, limiting the use of this method. Furthermore, it isnot possible for this attenuation to be switched off to exclude anattenuation by blood or air in the system. What the catheter personnelare accustomed to seeing makes them associate attenuation with aninadequately flushed system and they would easily misinterpret such anattenuated system.

The object of the present invention is to provide a method and a devicefor invasive pressure measurement with fluid-filled systems which areimproved with respect to the correction of measured valuefalsifications, are cost-effective and versatile in their use.

SUMMARY

The method according to the invention allows a correction of thepressure profile that is faithful with respect to phase and amplitude bythe evaluation and processing of the signal by means of digital Fourieranalysis, without using Fourier transformation of a signal of a fixedlength but instead working with variable signal lengths. In this way,the optimum segment length, with which a minimal error occurs, can bedetermined for the subsequent correction.

The correction method can be used for a wide variety of systems, therebyreducing the expenditure in financial terms and in terms of apparatusfor invasive pressure measurements. Furthermore, there are no longer anytype-dependent restrictions, so that the systems or catheters that areoptimum for the patient can be used, without having to dispense withcorresponding accuracy.

The output on various evaluating or indicating units permits rapid andcomprehensive evaluation of the data. A correction of the signals ispossible both online and offline.

For determining the segment length of the signal to be corrected, acomparison of the deviation of the inverse transform from the originalsignal is carried out on the basis of the variation of the length of abase signal. Starting from a prescribed base signal length, a comparisonof the inverse transform of the base signal with the original signal iscarried out. In this comparison, a deviation or error which changes independence on the chosen signal length is established. The signal lengthis then increased or reduced in steps, depending on which base signallength was taken as a starting point. If the error reaches a prescribedvalue, i.e. if a specific accuracy is achieved, the variation of thesegment length is discontinued in order to reduce the computationaleffort. An optimum segment length is found when the minimum of thedeviation of the inverse transform from the original signal has beendetermined.

It has been found to be favorable to start out in the variation of thebase signal length from a minimum length, which is increased in steps.If the deviation is reduced when the segment length is increased, thetransformation routine with error determination is repeated until theprescribed value for the deviation or the minimum is reached. Thesegment length or curve length found in this way is optimum for theFourier transformation of the correction method, since the measuredsignal can be broken down virtually completely into harmonicoscillations and the error is minimal. A value which is less than thelength of a heartbeat is to be assumed as the minimum length. A minimumsignal length of 0.3 seconds has been found to be a favorable value.

For faster determination of the optimum or prescribed value, the stepsize of the change in segment length change is varied in proportion tothe deviation of the inverse transform from the original signal. With asmall error, a correspondingly small change is made, since the segmentlength is already close to the optimum and the highest possibleresolution is aimed for by a small step size. With a large error, thesame applies in a correspondingly converse sense.

A variant of the method according to the invention allows a correctionof the pressure profile that is faithful with respect to phase andamplitude by the heartbeat-related evaluation and processing of thesignal by means of digital Fourier analysis. Other correction methodsare only inadequately able to take into account the differentfrequencies of the heartbeats.

In an advantageous refinement of the invention, the correction datadetermined on the basis of reference pressure measurements are called upfrom a matrix of correction data records, making a large number of datarecords available quickly and easily. To keep the number of empiricallydetermined correction data records to a commercially acceptable order ofmagnitude, if the exactly matching data record is missing aninterpolation is carried out between the closest data records.

To obtain a signal that is corrected as accurately as possible, a phasecorrection and amplitude correction are provided, it having proven to beadvantageous to carry out a phase correction of the signal only at thepoints where that signal has an amplitude.

For determining the correction data records, in one embodiment of theinvention the catheter tip is introduced into a device which can besubjected to pressure and this device is subjected to different mediumpressures and frequencies. In separate measurements, the medium pressureis varied in defined equidistant step sizes and the lowest frequency(fundamental frequency) of the frequency spectrum is likewise varied indefined equidistant step sizes. These settings produce a mediumpressure/frequency grid of coordinates, which represents the basis forthe correction data record matrix. As an alternative to this, thetransmission characteristic is determined by means of a white frequencynoise and the correction takes place by means of deconvolution of theoutput signal with the transmission function. A reference pressuremeasurement takes place with another measuring system, preferably with atip pressure sensor catheter.

It has been found to be favorable with respect to the computationaleffort and correction results for a defined signal in the form of afrequency grid to be used for determining the correction data recordsfor the system excitation. On the basis of a fundamental oscillation,which for computational reasons advantageously lies in the range between0.1 and 1 Hz, the system is excited with equidistant harmonicoscillations. From a fixed upper limit, the number of excitationfrequencies required is consequently obtained. 40 Hz has been found tobe a physiologically appropriate upper limit for the excitationfrequency.

In separate measurements, the medium pressure is varied in definedequidistant step sizes. These settings provide a set of correction datarecords for various medium pressures. A reference pressure measurementtakes place, as before, with another measuring system.

To achieve a coincidence of the spectral lines of the signal to becorrected with those of the correction data record vector, the pressuresignal segment is multiplied repeatedly until a ratio corresponding tothe resolution of the correction data record is obtained between thesampling rate and the length of the curve segment. If it does notcorrespond to the resolution, the next-smaller ratio between thesampling rate and the length of the curve segment is expediently set andthe assignment to the spectral lines of the correction data record takesplace by rounding up to the next corresponding line.

Since a pressure transducer generally does not emit an adequately strongsignal, an amplifier is provided between the pressure transducer and theanalog/digital converter. The pressure transducer is activated andsupplied with the required operating voltage by means of a supply lineof the signal processing and analyzing unit.

For the correction of the recorded signals in the heartbeat-relatedanalysis, it is very important that the length of the heartbeat isknown, since only in this way can a beat be processed exactly. Thelength of the beat is advantageously calculated by means of anautocorrelation function and its first derivative with respect to time.A prefiltering with a low-pass filter with a high cut-off frequency of30-40 Hz is optionally provided in order to eliminate possibleinterferences of the alternating current system.

For a reliable correction of the signals, it is necessary that thesignal analyzing and processing unit correctly assigns the respectivecorrection data records. Since the various systems are differentlydesigned, have different resonant frequencies and can be changedconsiderably by built-on parts, a system identification is carried outby means of a test signal response before the measured value isrecorded. A defined signal is preferably transmitted from a referencepressure transmitter (calibrator) at the tip of the catheter in thedirection of the pressure transducer and the system response is comparedwith experimentally found system responses. In this way, aclassification can be performed and information obtained on which systemis concerned or which correction data records are suitable for thesystem concerned. It is also conceivable for a signal to be transmittedfrom the pressure transducer in the direction of the catheter tip andthe signal response to be compared with experimentally found systemresponses.

It has proven to be advantageous in the heartbeat-related analysis forthe fundamental frequency to be determined by means of a combination ofa distribution analysis of maxima of autocorrelation functions ofvarying length with the analysis of the minima and maxima of the curve.In online determination of the fundamental frequency specifically, it isexpedient to repeat the autocorrelation function with an increasinglength and to collect all the first maxima of the autocorrelationfunctions of the increasing length. The most frequently occurringmaximum is subsequently determined by means of a distribution analysis.

A cross-correlation of the pressure signal and patient's ECG isadvantageously carried out to determine the length of the fluid-filledsystem, i.e. the signal delay time. In a variant, the systemidentification is carried out automatically.

In addition to a classification of the catheter and tube system, anidentification of the various pressure transducers and correspondingconsideration in the selection of the correction data records isadvantageously envisaged. Since the respective pressure transducersconvert the pressure signals differently, on a type- or model-dependentbasis, different operating voltages are required and have to beindividually activated, such an adaptation is advantageous in order tokeep the measured value falsification as low as possible and to carryout a correct activation.

As an additional check and in order to make use of the operator'sexperience, a manual interaction is envisaged in the systemidentification, so that a selection or input can be performed inaddition to or as a departure from the option calculated.

The system transmission properties of the fluid-filled system correlatewith the elasticity of the catheter and line system. Depending on thematerial properties, a different initial stress due to the averageinternal pressure prevailing in the system may therefore change thesystem transmission properties significantly. Continuous measurement ofthe medium pressure is therefore part of the automatic signal analysis.The selection of the correction data records takes place according tothe medium pressure.

For a reliable correction of the pressure signal it is advantageous ifso-called artifacts are detected. This takes place on the basis of thesystem identification determined. Excessive deviations are detected anddisregarded. In a variant of the method, in addition to the correctionof the pressure signal, an artifact identification and elimination iscarried out by means of brief autocorrelation. In the autocorrelationfunction, interference spikes in the pressure profile can beautomatically detected and localized. An interpolation of the curve atthe point of the spike eliminates the interference.

In addition to the correction instruments described, if appropriate, ashape analysis of the pressure signal may be carried out, taking higherharmonic fundamental oscillations into account, so that acorrespondingly refined method is available for compiling and selectingthe correction data records.

In a further refinement of the invention, an optional output of theuntransformed signal is envisaged, enabling the operator to detect themechanical attenuation due to blood clots or small air bubbles.

Static calibrating options which simplify operation, or supply moreeasily comparable results or signal profiles, are expediently provided.By zero point calibration, the measured pressure is assumed as the zeropoint and serves as a basis for the pressure monitoring system, whichindicates the signals. In this way, indications which can be comparedwith one another become possible without, for example, blood pressurefluctuations and system-related offsets between different measurementshaving to be taken into account. For checking the connection between thesignal analyzing and processing unit and the pressure monitoring systemand for checking the calibration, a reference pressure (for example 100mmHg, which can be set in the instrument menu) can be sent to thepressure monitoring system. In analogy with the reference pressure,various stored pressure curves may be sent as a test signal to thepressure monitoring system.

In a further embodiment of the invention, the signal is post-filtered orpost-corrected, in order to remove interference signals and to have asignal profile that is as unfalsified as possible. Such apost-correction is preferably carried out with respect to time on thebasis of the first derivative of the corrected and possibly smoothedpressure signal. For post-filtering, frequency or mean-value filters aresuitable.

In an advantageous refinement of the invention, an automatic adaptationto changes of the resonant response of the system as a result ofpressure changes is carried out. The changes in blood pressure may becaused, for example, by circulation-related reactions or medicaments,the changes in the resonant response being of a system-specific nature.The corresponding variables are determined continuously and arecontinuously fed to the signal analyzing and processing unit, whichtakes the changes into account in the selection of the correction datarecords.

A device according to the invention for carrying out the method has afluid-filled system for invasive pressure recording and a pressuretransducer, which converts the pressure pulses into electrical signals.Connected thereto is a recording unit for the original voltage signalsof the pressure transducer and analog/digital converter, which preparesthe signal for digital processing. In the signal analyzing andprocessing unit, which is designed for example as a computer, theindividual data records are provided with correction factors independence on the system parameters, on the basis of a digital Fourieranalysis, and are fed to an interface. The output unit processes thecorresponding signals, for example as an analog signal, as a digitalsignal, as a printout or as a display on a monitor.

The interface advantageously has a digital/analog converter, amplifiersand/or an adaptor, so that the corrected signal can be fed to amonitoring system, and can be transmitted to a computer in an amplifiedform and/or remaining in a digitized form.

A correction data record matrix, which contains correction factorsdetermined from experimental reference pressure measurements, isadvantageously stored in a memory of the signal analyzing and processingunit. In connection with the corresponding data processing programs, therespective or interpolated correction vectors can then be selected,interpolated if appropriate and combined with the digitized pressuresignal.

An attenuation is normally associated with an inadequately flushedsystem. To make use of previous experience, the device advantageouslyhas a signal output for the uncompensated signal, in order that theoperator has the possibility of comparing the corrected pressure signalswith the signals in pure form and in this way has a check on thecorrection method.

To take blood pressure fluctuations into account in the correctionmethod, in a development of the invention a device for their measurementis provided, the measured values determined having an influence on theselection of the correction data record.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis ofexemplary embodiments represented in the drawing, in which:

FIG. 1 shows a basic setup for the measured value correction,

FIG. 2 shows a basic setup for the compilation of the correction datarecord and

FIG. 3 shows a representation for determining the fundamental period.

DETAILED DESCRIPTION

FIG. 1 shows a basic setup of an invasive pressure measurement by meansof a fluid-filled system. In this arrangement, a so-called catheter 1 ismoved through the arterial or venous system of a patient into theproximity of the point at which the pressure is to be measured. For thepatient to be influenced as little as possible by the catheter 1, thelatter has the smallest possible dimensions. The catheter 1 itselfconsists of an elastic material and is of a tubular design. At the tipof the fluid-filled catheter 1 there is an opening, through whichpressure pulses are recorded and passed on through the catheter 1 and alikewise fluid-filled line 2 to a pressure transducer 3.

In dependence on the pressure pulses, the pressure transducer 3generates electrical signals, which can be correspondingly displayed orevaluated. This method has long been known in principle. A possiblecorrection of the transmission function of this system of the secondorder takes place after determining the resonant frequency and theattenuation coefficient by means of an analog electric circuit or acorresponding numerical algorithm.

For effective correction of the measurement falsifications occurringduring use of the method described above, which lie in the range of upto 40%, in the method according to the invention there is arrangedbetween the pressure transducer 3 and the signal analyzing andprocessing unit 5 an analog/digital converter 4, which converts theanalog signals of the pressure transducer 3 into digital signals, whichare applied to the input of the signal analyzing and processing unit 5.Within the signal analyzing and processing unit 5, the measured data aresubjected to correction factors on the basis of a digital Fourieranalysis and are passed on to the output or evaluating device 6.

Before the correction of the signals, an overall system identificationof the mechanical part of the system is carried out. Firstly, a manualor automatic identification of the connected pressure transducer 3 takesplace. Subsequently, a test signal in the form of a pressure pulse,preferably generated by a calibrator, is transmitted. As an alternative,the pulse generation is performed by a pressure transducer 3. Theparameters of the catheter-line system are determined from the signalresponse and a selection of the correction data records takes place onthe basis of said parameters. Since, with the large number of componentsused in invasive pressure measurement and the large number ofparameters, an exactly matching correction data record is not alwaysavailable, the values required are determined from the existing datarecords by means of interpolation methods and are provided for thecorrection.

The signals which have been digitized and subjected to corrected Fouriercoefficients are transmitted from the signal analyzing and processingunit 5 to an indicating or evaluating unit 6, it being possible for anindication to take place both on a monitor system and on a printout.Depending on the standard of the monitor, the signals are firstly fed toa digital/analog converter and subsequently output or transferreddirectly to a monitor which can process digital signal. If appropriate,the signals still have to be prepared in such a way as to provide aformat suitable for display.

Another possibility is for the data to be transmitted to a computer,which stores and evaluates them. In this case, the data are notprocessed in a digital/analog converter but are passed on directly fromcorrection.

There is also the possibility of not carrying out the correction onlinebut of storing the data and evaluating or correcting them at a laterpoint in time. A precondition for this is the presence of thesystem-specific data and the information on the measuring conditions, inorder that a correct selection of the correction data records cansubsequently take place. For this purpose, the data are advantageouslyrecorded directly after the pressure transducer 3 and stored on asuitable storage medium, for example a CD or floppy disk.

In a variant of the invention, an output capability for the uncorrectedsignal is provided, in order that there is the possibility of comparingthe corrected signals with the uncorrected signals. This has the effecton the one hand that what the operator is accustomed to seeing is notcompletely changed, and on the other hand that a check of the correctionmethod takes place. For example, the presence of air bubbles in thefluid-filled system can be detected from the uncorrected signal bytrained operators, so that corresponding measures can be taken. Thebranching of the signal may take place both before and after theanalog/digital converter 4, it being expedient for an amplifier to bearranged upstream, in order that an adequately strong signal isavailable.

Before the actual measurement, a calibration of the pressure to bemeasured with respect to atmospheric pressure is usually carried out,with a three-way cock that is usually provided on the pressuretransducer being actuated. Provided on the signal analyzing andprocessing unit 5 is an actuating element, with the actuation of whichthe pressure to be measured is assumed as the zero point and serves as abasis for further measurement and output.

For checking the connection between the signal analyzing and processingunit 5 and the output unit 6 and for checking the calibration, areference pressure signal or various stored pressure curves are sent tothe output unit 6. The deviation and the compensation to be carried outcan be determined from the difference between the setpoint signal andthe actual-value signal. If the entire measuring chain is to be checked,a reference pressure signal may be connected instead of a patient'spressure signal and, if appropriate, necessary offset and linearitycorrections can be carried out at the signal analyzing and processingunit 5 for each channel.

A basic setup for the empirical determination of the correction datarecords is represented in FIG. 2. For determining the natural dynamicsof a system, and consequently the correction data records, a tube 7 thatis filled with fluid and vented is used. At the tube 7 there arerespective connections 8 for filling, venting, the reference pressuremeasurement by means of a tip pressure sensor catheter 10 and theintroduction of the catheter (test system) as well as a device forpressure generation 9 (Biotek).

After introducing the tip of the catheter 1 into the proximity of thereference pressure measurement, the tube 7 is excited by a definedfrequency spectrum pressure. In separate measurements, the mediumpressure, usually in the range from 0 mmHg to 130 mmHg, is varied indefined equidistant step sizes. The frequency content of the excitationsignal is composed of a fundamental oscillation and a number of harmonicoscillations. The fundamental oscillation is usually 0.25 Hz and 160harmonic oscillations are excited, so that an upper frequency of 40 Hzis achieved by the equidistant intervals. It goes without saying thatother frequencies of the fundamental oscillation are possible, and inthe same way the number of harmonic oscillations can be varied. However,the values mentioned represent an appropriate selection.

The Fourier spectrum of the reference signal and of the fluid pressuresignal is calculated from each measurement by means of Fouriertransformation. The correction data record vector is then obtained fromthe complex division of each spectral line of the reference pressure bythe corresponding spectral line of the fluid pressure. The result is aunitless, complex correction factor for each spectral line of thismeasurement. All the measurements together produce the correction datarecord matrix for the system investigated, which are stored in thesignal analyzing and processing unit 5.

In the heartbeat-related analysis, the length of the fundamentaloscillation in the invasive pressure measurement corresponds to aheartbeat, it being possible for the heart rate to change considerablyfrom beat to beat. A continuous analysis of the fundamental frequency istherefore part of the automatic signal analysis and is determined by anautocorrelation function.

In the segmental analysis of the signals recorded, different segmentlengths are used, obtained from a comparison of the inverse transformwith the original signal, with the segment lengths expediently beingchosen such that there is a minimal error, which according to experiencelies around 1%. This means that a segment length that is optimal for thecorrection method has been determined.

The number of Fourier coefficients, and consequently the correction datarecord, are consequently dependent on the length of the analyzed segmentor the fundamental frequency.

In addition to the determination of the fundamental frequency or thesegment length, the medium pressure is a variable to be determined. Thesystem transmission properties of the fluid-filled system are dependent,inter alia, on the elasticity of the catheter and line system 1, 2.Depending on the material properties, a different initial stress due tothe average internal pressure prevailing in the system can thereforechange the system transmission properties significantly. A continuousmeasurement of the medium pressure is therefore likewise part of theautomatic signal analysis. The selection of the correction data recordstakes place according to the medium pressure.

Before the beginning of the actual correction, the signal may befrequency-filtered, optional use being envisaged for the numericalfilter, as well as a variation of the filter cut-off frequency, whichaccording to experience lies between 40 and 100 Hz. Such filtering maybe necessary, for example, in the event of interferences caused by the50 Hz alternating current system.

To characterize the signal for the correction, the fundamental frequencyor the segment length and the medium pressure are required.

An example of the determining of the fundamental period is representedin FIG. 3. For this purpose, firstly the autocorrelation function (ACF)is calculated. The time until the occurrence of a main maximum exceedinga threshold value is the fundamental period.

On the basis of the level of the medium pressure, the correspondingcorrection data record is selected. The medium pressure is obtained fromthe normalized level of the first spectral line (line of the frequencyzero, direct component) of the Fourier transform of the signal.

In addition, a possible dependence of the transmission characteristic ofthe system on the frequency content of the exciting signal is counteredby a simple shape analysis of the signal, based on higher harmonicfundamental oscillations with corresponding modification of thecorrection data records.

In a preferred alternative, the complex Fourier coefficients of thepressure signal are then multiplied by the complex correctioncoefficients of the selected correction factor. In a way similar to inthe compilation of the correction data records, in the heartbeat-relatedanalysis it is also the case for the pressure signal that thefundamental frequency and its harmonic oscillations are corrected onlywhere they exceed a threshold value, up to an upper frequencycorresponding to the highest frequency of the correction data records,in the present case 40 Hz. All other frequency components are set tozero.

The multiplication produces the corrected Fourier spectrum of thepressure signal, that is then inversely transformed into the correctedpressure signal by means of inverse discrete Fourier transformation.

In another embodiment of the invention, in the heartbeat-relatedanalysis the variables of fundamental frequency and medium pressure areused for selecting the corresponding correction data record from thecorrection data record matrix. If the position of the measurement doesnot lie exactly at a coordinate point of the matrix, all thecoefficients are newly calculated with a weighted interpolation from theneighboring coefficients.

The reciprocal value of the fundamental frequency, the fundamentalperiod, determines the number of points for the subsequent discreteFourier transformation of the pressure signal, the segment to becorrected being doubled or multiplied as required for the Fourieranalysis. The complex Fourier coefficients of the pressure signal arethen multiplied by the complex correction coefficients of the selectedor interpolated correction vector.

To achieve a coincidence of the spectral lines of the pressure signal tobe corrected with those of the correction data record vector, thepressure signal segment (here a heartbeat) is multiplied repeatedlyuntil the ratio which corresponds to the resolution of the correctiondata record is obtained between the sampling rate and the length of thecurve segment.

If, for example, correction coefficients for the frequencies of 0.25 Hz,0.50 Hz, 0.75 Hz, . . . 40 Hz (spectral resolution of 0.25 Hz) arepresent, at a sampling rate of 1000 Hz the curve segment of the pressuresignal must contain at least 4000 points, since then a ratio of thesampling rate to the length of the curve segment of ¼ is obtained(<=>0.25 Hz). If this ratio cannot be exactly achieved, the next-smallerratio (<¼) is set. The assignment to the spectral lines of thecorrection data record then takes place by a rounding up to the nextcorresponding line.

For determining the fundamental frequency, a distribution analysis ofmaxima of autocorrelation functions of varying length is combined withthe analysis of the minima and maxima of the curve.

In the method for the online determination of the fundamental frequencyof a pressure signal, the fundamental frequency is calculated by meansof an autocorrelation function (ACF). In this case, the number offunction values up to the first main maximum corresponds to the lengthof the heartbeat, in other words the reciprocal value of the fundamentalfrequency. Since, in online operation, the number of measured values issmall at the beginning and increases with time, the ACF is repeated withan increasing length. This gives rise to the problem that a considerablychanged second heartbeat strongly influences the result. For the optimumdecision as to when the length of the heartbeat has been correctlydetermined, all the first maxima of the ACFs of increasing length arecollected and the maximum which occurs most frequently is selected bymeans of a distribution analysis.

In a segmental analysis of the measured signal it is possible todispense with a determination of the fundamental frequency by means ofautocorrelation. For determining the segment length, the complex Fourierspectrum is calculated for a minimum length, for example 0.3 seconds, ofthe digitized pressure signal. The frequency components above a fixedlimit, which is determined by the highest frequency of the correctiondata records, in the present case 40 Hz, are set to zero. Subsequently,the spectrum is transformed back into the time domain and compared pointby point with the original curve. The comparison gives a deviation witha specific value. The length of the investigated segment is thenincreased in steps and the transformation, the frequency filtering,inverse transformation and deviation determination are repeated until aminimum of the deviation has been found. The segment length determinedin this way is optimal for the Fourier transformation of the correctionmethod, that follows the segment length determination.

A signal sampled at 1000 points per second for the correction datarecord is treated with a 4000-point Fourier transformation. This gives:

f ₁=0 Hz, f ₂=0.5 Hz, etc. up to f _(n)=40 Hz

If the curve segment to be corrected is likewise sampled at 1000 pointsper second and the fundamental period is 1000 points long, thefrequencies of the Fourier transformation are obtained as:

 h ₁=0 Hz, h ₂=1 Hz, h ₃=2 Hz etc. up to h _(m)=999 Hz

To be able to apply the 160 points of the Fourier transformation of thecorrection data record to the 1000 points of the curve segment whileretaining a steadily progressing pressure profile, the correspondingfrequency lines up to 40 Hz are used for the correction and aremultiplied by the values of the curve segment. All the other frequencylines are set to zero. The multiplication produces the corrected Fourierspectrum of the pressure signal, which is then inversely transformedinto the corrected pressure signal by means of inverse discrete Fouriertransformation.

For post-processing operations, as in the case of the signal input, theoutput signal can likewise be frequency-filtered. The numerical filtermay be optionally switched on and off by the user and the filter cut-offfrequency can be varied. A signal improvement is also achieved by amean-value filtering connected to the frequency filtering, for which afreely configurable mean-value filter (moving average filter) with alength of 2 to 20 points is provided. These filters can also be switchedon and off. To improve the correction result, an additional correctionmay be switched on, which adds or subtracts to or from the correctedsignal, on a point-by-point basis, the first derivative with respect totime, displaced by n points.

What is claimed is:
 1. A method for the correction of measured valuefalsifications in invasive pressure measurements with a fluid-filledsystem, the method comprising: passing measured pressure, as a pressuresignal, via the fluid-filled system to an external pressure transducer;converting the pressure signal; into an electrical signal digitizing theelectrical signal in an analog/digital converter; feeding the digitizedsignal to a signal analyzing and processing unit, which operates on thebasis of a Fourier analysis; analyzing the digitized signal in segments;combining the analyzed signal with prescribable correction data in theform of Fourier coefficients, the analyzed signal being at least onesegment of the digitized signal; varying length of the segments, suchthat a minimal error occurs in the Fourier analysis and the digitizedsignal is corrected by the signal analyzing and processing unit; andpassing the corrected signal to at least one of an output and evaluatingunit.
 2. The method according to claim 1 wherein the length of thesegments is determined by a variation of length of a base signal and acomparison of an inverse transform of the base signal with theelectrical signal, deviation of the inverse transform of the base signalfrom the electrical signal assuming a prescribed value.
 3. The methodaccording to claim 2 wherein the length of the segments is determined bya minimum of the deviation of the inverse transform of the base signalfrom the electrical signal.
 4. The method according to claim 2 or 3wherein varying the length of the segments is performed in steps,starting from a base signal length.
 5. The method according to claim 4wherein varying the length of the segments is increased in steps,starting from a minimum length.
 6. The method according to claim 4wherein the steps have a size proportional to the deviation of theinverse transform of the base signal from the electrical signal.
 7. Amethod for the correction of measured value falsifications in invasivepressure measurements with a fluid-filled system, the method comprising:passing measured pressure, as a pressure signal, via the fluid-filledsystem to an external pressure transducer; converting the pressuresignal into an electrical signal; digitizing the electrical signal in ananalog/digital converter; feeding the digitized too signal analyzing andprocessing unit, which operates using digital Fourier analysis;analyzing the digitized signal on a heartbeat-related basis; combiningthe digitized signal analyzed on the heartbeat-related basis withprescribable correction data in a form of Fourier coefficients, with afundamental frequency being determined by means of an autocorrelationfunction and a first derivative with respect to time of theautocorrelation function, such that the digital signal is corrected bythe signal analyzing and processing unit, and passing the correctedsignal to at least one of an output and evaluating unit.
 8. The methodaccording to claim 7, wherein the correction data are called up from acorrection data record matrix of correction data records.
 9. The methodaccording to claim 8 wherein the correction data are called up as acorrection data record vector.
 10. The method according to claim 7further comprising performing a phase and/or amplitude correction of theanalyzed signal.
 11. The method according to claim 10 wherein the phasecorrection of the analyzed signal takes place only at the points wherethe analyzed signal has an amplitude.
 12. The method according to claim8 further comprising performing an interpolation between the correctiondata records.
 13. The method according to claim 8 wherein the correctiondata record matrix is determined by introducing the fluid-filled systeminto a device which can be subjected to pressure and the device issubjected to different medium pressures and frequencies, such that areference pressure measurement is performed using a different measuringsystem and harmonic analysis is carried out.
 14. The method according toclaim 8 further comprising determining correction data records using afrequency grid for system excitation.
 15. The method according to claim14 wherein the frequency grid is based on a fundamental oscillation andharmonic oscillations.
 16. The method according to claim 15 wherein thefundamental frequency lies between 0.2 Hz and 3 Hz.
 17. The methodaccording to claim 16 further comprising exciting a corresponding numberof harmonic oscillations until a fixed upper limit is reached byequidistant intervals.
 18. The method according to claim 17 wherein theupper limit is 40 Hz.
 19. The method according to claim 9 furthercomprising multiplying repeatedly a pressure signal segment until aratio which corresponds to a resolution of a correction data record isobtained between sampling rate and a length of a curve segment, in orderto achieve a coincidence of spectral lines of the digitized signal to becorrected with those of the correction data record vector.
 20. Themethod according to claim 19 further comprising: setting a next-smallerratio between the sampling rate and the length of the curve segment; andassigning to the spectral lines of the correction data record byrounding up to a next corresponding line.
 21. The method according toclaim 13 further comprising: determining transmission characteristic ofthe fluid-filled system by means of white frequency noises; andperforming a correction by means of deconvolution of an output signalwith a transmission function.
 22. The method according to claim 7further comprising amplifying the electrical signal from the pressuretransducer.
 23. The method according to claim 7 further comprisingsupplying the pressure transducer with voltage by the signal analyzingand processing unit.
 24. The method according to claim 7 wherein thefundamental frequency is determined by means of an autocorrelationfunction and the first derivative with respect to time of theautocorrelation function.
 25. The method according to claim 19 whereinthe fundamental frequency is determined by means of a combination of adistribution analysis of maxima of autocorrelation functions of varyinglength with analysis of minima and maxima of the curve segment.
 26. Themethod according to claim 25 further comprising performing onlinedetermination of the fundamental frequency by repeating theautocorrelation functions with an increasing length, collecting allfirst maxima of the autocorrelation functions of the increasing lengthand determining a most frequently occurring maximum by means of adistribution analysis.
 27. The method according to claim 7 furthercomprising determining the ECG signal delay time by performing across-correlation of the pressure signal and a patient'selectrocardiogram (ECG).
 28. The method according to claim 7 furthercomprising performing a system identification by means of a test signalresponse.
 29. The method according to claim 28 wherein the systemidentification is performed automatically.
 30. The method according toclaim 28 or 29 further comprising performing a manual interaction in thesystem identification.
 31. The method according to claim 28, furthercomprising performing a continuous measurement of medium pressure. 32.The method according to claim 28 further comprising generating a testsignal by the pressure transducer.
 33. The method according to claim 28further comprising generating a test signal by a calibrator.
 34. Themethod according to claim 7 further comprising adapting a pressuretransducer for the fluid filled system.
 35. The method according toclaim 7 further comprising identifying of artifacts on a basis of systemidentification, harmonic base frequency and signal medium pressure. 36.The method according to claim 35 further comprising: performing anartifact identification and elimination by means of an autocorrelationfunction; and smoothing interference by an interpolation of measuredvalues at a point of a spike.
 37. The method according to claim 7further comprising performing an analysis of the fundamental frequency,medium pressure and shape of the pressure signal by means of harmonicanalysis.
 38. The method according to claim 37 further comprisingperforming a correction taking into account higher harmonic fundamentaloscillations.
 39. The method according to claim 7 further comprisingoutputting an untransformed signal.
 40. The method according to claim 7,further comprising performing a calibration of the system using a zeropoint calibration, a reference pressure measurement and/or a testsignal.
 41. The method according to claim 7 wherein the electricalsignal is post-filtered and/or post-corrected.
 42. The method accordingto claim 41 wherein the post-correction of the electrical signal iscarried out on basis of a first derivative of the pressure signal withrespect to time.
 43. The method according to claim 42 wherein thepost-filtering is carried out by means of a frequency and/or mean-valuefilter.
 44. The method according to claim 7, further comprisingperforming an automatic adaptation to changes of resonant response as aresult of pressure changes.
 45. A device for the correction of measuredvalue falsifications in invasive pressure measurements of a fluid-filledsystem, the device comprising: a pressure transducer converting recordedpressures into electrical signals; an analog/digital converter,connected to the pressure transducer, digitizing the electrical signals;signal analyzing and processing unit connected to the analog/digitalconverter; and an output or evaluating unit connected to the signalanalyzing and processing unit; wherein the signal analyzing andprocessing unit has a means for carrying out a Fourier analysis, whichbreaks down the digitized signal into segments and subjects thesegmented signals to a Fourier analysis, the segmented signals beingcapable of being combined with prescribable correction data in a form ofFourier coefficients and length of the segments being varied such that aminimal error occurs in the Fourier analysis.
 46. The device accordingto claim 45 further comprising an interface designed as a digital/analogconverter, amplifier and/or adaptor.
 47. The device according to claim45 or 46 wherein the signal analyzing and processing unit includes amemory and a correction data record matrix is stored in the memory ofthe signal analyzing and processing unit.
 48. The device according toclaim 45 further comprising a signal output for an uncorrected signal.49. The device according to claim 45 further comprising a devicemeasuring blood pressure fluctuations.